The present invention relates to a circuit for controlling two signals approximately 90.degree. apart in phase so as to obtain two signal waveforms having a phase difference of exactly 90.degree. and exactly equal amplitudes, as set forth in the preamble of Claim 1. A control circuit of this kind is described in Offenlegungsschrift DE-A-33 13 867. FIG. 1 and the description thereof on page 8, line 1, to page 11, line 6, serve to explain this control circuit, which includes a phase control stage and an amplitude control stage. The control circuit forms part of a superheterodyne receiver which converts a received composite radio-frequency signal--in that case a composite color signal--directly to the baseband with the aid of a radio-frequency quadrature mixer using the "third method", with the upper sideband and the lower sideband being placed one upon another. By means of an audiofrequency quadrature mixer, the composite signal is converted to the usual AF value, the undesired sideband being suppressed; cf. "Proc. IRE", December 1956, pages 1703-1705, "A Third Method of Generation and Detection of Single-Sideband Signals".
In such circuits, especially if they are used to convert the transmitted composite television signal in accordance with the PAL or NTSC television standard, an exact 90.degree. phase difference and amplitude identity are required for the two quadrature-signal paths, namely the paths for the first and second quadrature analog signals, respectively, because otherwise picture disturbance and audio interference would result.
A disadvantage of the prior art control circuit is that the phase control stage acts on the sensitive 90.degree. phase shift of the radio-frequency oscillator in the radio-frequency quadrature mixer. Another disadvantage is that the phase deviation is determined by a phase comparison in the baseband, i.e., at low frequencies, while the phase adjustment is performed on the highfrequency side, which is unfavorable from a control point of view because of the great frequency difference in the direct conversion technique. A further disadvantage of the prior art control circuit is that the amplitude control stage responds only to pure amplitude changes. By contrast, the amplitude identity of the two signal paths should relate to the amplitude identity of equal frequency components of each of the two quadrature analog signals.